When such a resistor is traversed by current, predetermined fractions of the overall voltage difference existing between its terminals are available at the several taps. The latter may be constituted by metallic strips placed across the doped zone of the substrate at longitudinally spaced locations. Another possibility is to extend lateral branches from that zone by diffusing impurities--usually identical with those of the zone itself--into adjacent areas and metallizing the extremities of these branches. If the taps thus formed are connected to loads of high resistance (such as, for example, the inputs of operational amplifiers), they will draw only negligible current so as not significantly to affect the overall voltage drop.
In certain instances it is necessary to make the open-circuit potential difference between adjacent taps a very small fraction of that overall voltage drop. Such a situation arises, for example, when a reference potential is to be applied to a resistance bridge requiring compensation of an offset voltage due to an inherent unbalance. It then becomes necessary to choose among several reference voltages differing but little from one another, e.g. by about 1 to 3 mV.
For physical reasons, metal contacts overlying a diffused resistor defined by an elongate zone of predetermined conductivity cannot be spaced close enough to supply voltage differences of this small order of magnitude. The same is true of taps formed by lateral branches extending on one side of the longitudinal median of the zone. If these branches are disposed at opposite sides of the median, their potential difference in the presence of a constant current of a given magnitude could be chosen as small as desired; such positioning, however, is inconvenient in certain instances, as where the selected tap is to be linked to an external load by a lead of predetermined length.